1. Field of the Invention
The present invention relates to a sliding element for seals used such as for mechanical seals and to their process of manufacturing. More particularly, the invention relates to a sliding element made from carbon having high degree of hardness which is used for mechanical seals under a heavy load condition, and which reduces the damage caused by cutting wear with floating solid foreign matters in sealed fluid or cavitation erosion of sliding material.
2. Description of the Related Art
Generally, carbon sliding materials are often used as seal rings for mechanical seals to seal the fluid. Between seal sliding faces of the carbon sliding material and that of mating sliding material, when solid foreign matters such as molding sand, solder or cutting powder are tramped, floated and invaded in the sealed fluid from each parts of the system where seals are equipped, the solid foreign matters causes cutting wear on sliding face of the carbon sliding material. This may lead to the damage such as extraordinary surface roughness or extraordinary wear on sliding face and consequently, sealed fluid may leak in cases.
Also, when the mechanical seals are used under a heavy load condition, by sliding, sliding faces are mirror surfaced and become in boundary lubrication condition. Lubrication film between the sliding faces become extremely thin and viscous drag become high, accordingly, Blister effect (surface blister/abrasion) on carbon materials may occur. Also, fluid located to a neighborhood of sliding face may show intense movement and there are cases when neighborhood of mechanical seals become negative pressure and cavitation erosion occur on carbon sliding materials. Consequently seal function may not stably be maintained.
For above mentioned mechanical seal problems, recently, as sliding materials, the combination of silicon carbide sliding materials that show high degree of hardness and excellent wear property may be used.
However, silicon carbide material is poor in self-lubricant property compared to carbon material. Accordingly, by sliding under a heavy load condition, sliding faces become mirror surfaced, fluid film between the sliding faces can not be maintained and become in boundary lubrication, and along with the effect of sliding heat, the sliding faces may be secured to each other and seizure may occur in cases. And due to the sliding heat, secondary seal portion of rubber packing or the like may be damaged and sealed fluid may leak, consequently seal function may not stably be maintained. Further, sliding of the two silicon carbide materials, for solid lubrication can not be expected, if, with any reason, lubrication fluid does not exist in an atmosphere of seal even for a short time, with the sliding heat, the temperature near the sliding face show sudden rise and the same inconvenience occur in cases. At the time of initial running or deactivation of sliding, lubrication film between the sliding faces become thin and sliding torque may inconveniently rise extraordinarily.